This invention relates generally to communication systems and, more particularly to interleavers for performing code modulation.
Techniques for encoding communication channels, known as coded modulation, have been found to improve the bit error rate (BER) of electronic communication systems such as modem and wireless communication systems. Turbo coded modulation has proven to be a practical, power-efficient, and bandwidth-efficient modulation method for xe2x80x9crandom-errorxe2x80x9d channels characterized by additive white Gaussian noise (AWGN) or fading. These random-error channels can be found, for example, in the code division multiple access (CDMA) environment. Since the capacity of a CDMA environment is dependent upon the operating signal to noise ratio, improved performance translates into higher capacity.
An aspect of turbo coders which makes them so effective is an interleaver which permutes the original received or transmitted data frame before it is input to a second encoder. The permuting is accomplished by randomizing portions of the signal based upon one or more randomizing algorithms. Combining the permuted data frames with the original data frames has been shown to achieve low BERs in AWGN and fading channels. The interleaving process increases the diversity in the data such that if the modulated symbol is distorted in transmission the error may be recoverable with the use of error correcting algorithms in the decoder.
A conventional interleaver collects, or frames, the signal points to be transmitted into an array, where the array is sequentially filled up row by row. After a predefined number of signal points have been framed, the interleaver is emptied by sequentially reading out the array column by column for transmission. As a result, signal points in the same row of the array that were near each other in the original signal point flow are separated by a number of signal points equal to the number of rows in the array. Ideally, the number of columns and rows would be picked such that interdependent signal points, after transmission, would be separated by more than the expected length of an error burst for the channel.
Non-uniform interleaving achieves xe2x80x9cmaximum scatteringxe2x80x9d of data and xe2x80x9cmaximum disorderxe2x80x9d of the output sequence. Thus the redundancy introduced by the two convolutional encoders is more equally spread in the output sequence of the turbo encoder. The minimum distance is increased to much higher values than for uniform interleaving. A persistent problem for non-uniform interleaving is how to practically implement the interleaving while achieving sufficient xe2x80x9cnon-uniformity,xe2x80x9d and minimizing delay compensations which limit the use for applications with real-time requirements.
Finding an effective interleaver is a current topic in the third generation CDMA standard activities. It has been determined and generally agreed that, as the frame size approaches infinity, the most effective interleaver is the random interleaver. However, for finite frame sizes, the decision as to the most effective interleaver is still open for discussion. Decreasing the amount of memory space (RAM or ROM) needed to store the information required to carry out an interleaving scheme is also a subject of current discussion.
Accordingly there exists a need for systems and methods of interleaving codes that improve non-uniformity for finite frame sizes.
There also exists a need for such systems and methods of interleaving codes which are relatively simple to implement, including having relatively low memory space requirements.
It is thus an object of the present invention to provide systems and methods of interleaving codes that improve non-uniformity for finite frame sizes.
It is also an object of the present invention to provide systems and methods of interleaving codes which are relatively simple to implement and which have relatively low memory space requirements.
These and other objects of the invention will become apparent to those skilled in the art from the following description thereof.
The foregoing objects, and others, may be accomplished by the present invention, which includes an interleaver for interleaving these data frames. The interleaver includes a storage area containing an array large enough to store the largest expected data frame. A frame of size L elements to be interleaved is stored in Nr(l) rows and Nc(l) columns of the array, where Nr(l) is a predetermined integer and Nc(l) is a prime number which satisfy the inequality
Nr(l)xc3x97Nc(lxe2x88x921) less than L less than Nr(l)xc3x97Nc(l)
where Nc(lxe2x88x921) is the highest prime number less than Nc(l). The elements of each row are permuted according to a predetermined mathematical relationship, and the rows are permuted according to predetermined mapping.